1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to high energy lasers.
2. Description of the Related Art
High energy lasers are currently being evaluated for military and industrial applications. For these applications, fiber lasers have been shown considerable promise. These lasers are somewhat simple in design and have achieved relatively high power levels in the kilowatt range. A laser includes a laser amplifier mounted within a laser resonator. The resonator provides the feedback necessary to build oscillation within the laser. The bulk laser active medium of the laser amplifier may be in the shape of a slab, rod or disk. In a fiber laser, the active medium is an optical fiber doped with active ions and serves as the active medium for the laser. When pumped, the medium provides amplification. The provision of reflective gratings at the ends of the active medium provides a resonator.
The active medium in conventional fiber lasers is somewhat small, i.e., typically on the order of 10 microns which limits power scalability. This is due to the requirement of a small diameter to sustain a diffraction limited mode of operation within the laser. While efforts have been made to increase the core diameter of fiber lasers, to date, these efforts have not been sufficiently successful to raise the power limitations of fiber lasers to the levels needed for current and future applications.
For example, high energy solid-state lasers (based on Yb and Nd solid-state materials operating near 1 micron wavelengths) are gaining ground in the development toward achieving weapons grade status. One promising approach is based on the Yb:glass fiber lasers: Unfortunately, the highest power levels achieved to date are close to the damage threshold of the single mode fibers employed in these systems.
This has lead to attempts to coherently combine the outputs of multiple oscillators to achieve higher output power levels including the use of a Talbot effect and active modulation feedback loops. Although phase locking of multiple core fiber (MCF) lasers has been demonstrated, these systems rely on the Talbot effect (based on periodical structures) to phase lock. This phase locking technique is limited as it requires extremely precise equalization of the lengths of the individual cores and the positioning of the cores in precisely periodical structure and is also dependent on the individual power output in each core.
Further, there is a limit with respect to the number of elements that can be locked in this way because of multiplexing issues in the common cladding. That is, to tens of elements in a MCF type configuration, the Talbot effect would suffice. However, to extend the scaling to multi-kilowatt powers would require separate fiber laser oscillators. In that case, the Talbot effect by itself would not possess the required robustness to perform an effective phase locking.
An alternative approach previously explored involves the use of an active feedback loop for digital wavefront measurement and wavefront control. In this case, individual fiber amplifiers are used instead of individual fiber lasers. The output of a single oscillator is split and launched into individual fiber channels. At the output, the entire wavefront of the fiber amplifier array is measured and used to control a phase control element. However, this active method is limited to a few oscillators and quickly becomes too complex to implement for larger numbers of oscillators (i.e., greater than 10).
Thus, there is a need in the art for an improved system or method for coherently combining the outputs of multiple individual fiber lasers.